A large number of psychiatric (i.e. schizophrenia), neurological (i.e. Parkinson's disease), and neurodegenerative (i.e. Huntington's chorea) pathologies involve changes of mental states or conditions based upon changes in neurotransmitter and receptor balances. Detection of such changes may allow for diagnosis well ahead of manifestation of severe clinical symptoms, and knowledge of the nature and the extent of such changes is of paramount importance for the determination of therapy. For instance, in Parkinson's disease the chronic use of L-DOPA therapy leads to a progressive diminution in its efficacy. Thus, one would like to be able to monitor the progression of the disease more closely to effect possible changes in dosing. Similar problems present for many of the currently used dopaminergic ligands in schizophrenia. Determination of the effects of these therapies upon the brain is very difficult at the present time.
The brain provides especially illustrative examples because of various difficulties in accessing the tissue directly during life. In a broader sense, though, many biological questions reduce to “what is the sensitivity of this tissue, or this subject, to a given drug or to other manipulation of a given receptor?”
Some existing methods allow identification of a region of tissue that is especially sensitive to a given pharmacological agent (“mapping”), but do not characterize that response across a range of doses or quantify the sensitivity. This has uses but usually fails for comparisons across long stretches of time or between subject groups.
Two methodologies have been widely used for the determination of changes in neurotransmitter and receptor dynamics in vivo. These two techniques (Positron Emission Tomography and Single Photon Emission Computed Tomography, PET and SPECT) involve the use of radioactivity. Positron Emission Tomography is a very versatile technique which has been used successfully for the mapping and sometimes quantification of Cerebral Blood Flow (CBF), cerebral glucose metabolism (using [18F]fluorodeoxyglucose, FDG) or receptor activity (using radioactive pharmacological ligands), while SPECT has until recently been more limited to the detection of nonspecific processes. Unfortunately, both techniques suffer from severe limitations in spatial and temporal resolution, and cannot generally be proposed for repeated applications. Measuring receptor sensitivity quantitatively with PET or SPECT often requires invasive steps such as arterial catheterization. Moreover, PET is characterized by limited availability and high costs, which are partly due to the short half-life of many of the cyclotron-produced radiopharmaceuticals which have to be administered.
A number of existing strategies allow quantification of pharmacological sensitivity in vivo. Traditionally this is done by measuring a biological signal after repeated experiments, each performed at a specific drug dosage, across a wide dose range. The resulting data can be plotted to show a dose-response curve (see FIG. 1). Signals can be quite varied and include clinical information such as blood pressure or subjective emotional state, endocrine responses, measurement of chemicals in tissues or body fluids, volume of an organ, cell number or shape, staining and microscopic examination, or measurement of electrical signals such as electrocardiogram (EKG) or single-neuron electrical recordings.
The area of pharmacology that describes and interprets such information is called pharmacodynamics. Traditionally, data from dose-response curves are fit to a mathematical model of drug-receptor interactions. These pharmacodynamic (PD) models allow characterization of the dose-response curve's shape, and position along the dose axis, by a small number of model parameters. Common models used to describe drug-receptor interactions return parameters such as the Bmax and KD, often used in tracer kinetic studies, or the Emax and EC50 or ED50, more commonly reported in clinical studies. The parameter Emax refers to the maximum possible magnitude of a measured effect even at high doses of drug. The concentration of drug in blood or another body fluid that produces an effect half the magnitude of Emax is denoted EC50. Instead of blood concentration one can characterize the administered dose of drug similarly; a dose of size ED50 produces an effect of magnitude Emax/2.
All these solutions to calculating quantitative pharmacodynamic information are limited in important ways. These include (a) spatial limitations (such as with needle electrophysiological recordings, microdialysis, or measurements of systemic, e.g. clinical or endocrine, responses; (b) limitations to interpretation at one cellular level, e.g. receptor quantification by PET does not provide a measurement of the sensitivity of the whole complex of receptor and postsynaptic cell(s); and (c) limited pools of subjects, e.g. with autopsy, biopsy, PET, microdialysis, or intrasurgical recordings. Further, PET or SPECT measurements of specific receptors can proceed only after extensive testing of each specific radioligands, including tests of safety, drug distribution including penetration of the organ of interest, radiodosimetry, displaceability, and pharmacological specificity.
An alternative approach has been called pharmacological challenge imaging. In this approach a receptor-effector system is manipulated by administration to the organism of a specific pharmaceutical agent, such as a synthetic agonist at a receptor for an endogenous transmitter or hormone. Additionally, an imaging device, records a response of tissue to this agent. Generally in this case the response, such as glucose or oxygen metabolism, or blood flow, is a general reflection of tissue activity and can also be observed in response to varied other physiological stimuli (such as sensory stimulation, mental work, or physical exercise). Pharmacological challenge imaging has several potential specific advantages: the entire system of receptor and effector mechanisms is probed simultaneously, responses can be measured in numerous locations simultaneously (e.g., across the entire brain), and responses can be seen in numerous regions all of which may be affected by receptor stimulation in the whole organism.
Pharmacologic challenge imaging began with ex vivo rodent studies with autoradiographic studies of local blood flow or metabolism in tissue slices. In the past several decades PET and SPECT have also been used to observe changes in humans and other animals. However, only rarely have quantitative measures been used or a wide range of dosages administered so that one could adequately characterize the dose-response curve to the drug.
More recently, functional magnetic resonance imagine (fMRI) has been used as the imaging method in pharmacological challenge imaging. This application has been dubbed pharmacological Magnetic Resonance Imaging (phMRI). The most common fMRI technique is sensitive to changes in blood oxygen level (Blood Oxygen Level Dependent signal, BOLD). Unfortunately most BOLD-sensitive fMRI is nonquantitative. This implies that responses to drug cannot be determined over a wide range of drug doses. Additionally, quantitative comparisons of drug response over time or between groups in an experiment are largely impossible. The situation is somewhat analogous to measuring distance with a stretchable ruler—it may be useful over short periods of time or in certain clever experimental designs, but one would view with suspicion an apparent difference obtained by measurements from the stretchable ruler taken in two different settings.
Other investigators have sought to circumvent the “stretchable ruler” problem of BOLD-sensitive fMRI by adding additional experimental data. Some groups have used what could be called a pure pharmacokinetic (PK) approach. In this approach, blood levels of drug after a single dose of drug are computed repeatedly over time from a subject or from a group of subjects. The resulting time-concentration curves are used to constrain a search for imaging signals that correspond in time to the expected blood concentrations of the dose of drug. The general plan is to observe changes that occur rapidly in comparison to the more gradual artifactual changes in the nonquantitative signal: by analogy, to measure quickly, before the ruler stretches too much. One serious problem with this method is that it assumes that tissue response is a linear function of drug when in fact two low doses may have indistinguishable or negligible effect and two higher doses may each have maximal effect (FIG. 1). Another problem with this method is that only drugs with rapid onset and rapid removal from the system are suitable. While such drugs exist, e.g. injected nicotine or inhaled cocaine, the reality is that many prescription drugs of interest are selected especially not to have these characteristics. Patients prefer to take only one dose of drug a day, so slow drug elimination is often preferred. Since many side effects are concentration-related, slower absorption and consequently lower peak concentrations are also desired.
Another attempt to circumvent limitations of BOLD fMRI for pharmacological imaging involves what might be called pure pharmacodynamic phMRI. In this method a clinical parameter, such as pain relief by an opiate or a feeling of “high” induced by cocaine, is measured repeatedly over time either in the scanner or during a separate experimental session. The resulting time-response curve is used to constrain a search for imaging signals that correspond in time to the expected clinical effect of the drug. Safety or ethical issues aside, this approach suffers from specific problems including (a) the imaging signal observed cannot easily be interpreted as being a direct effect of drug, rather than an indirect effect of pain relief or feeling “high”; (b) only drugs with an easily detectable clinical effect can be used; and (c) studies of a medicament used for a specific disease will usually require studies only in patients, rather than in control groups, since the control subjects have by definition normal function not expected to improve detectably with administration of drug.
Newer imaging methods may overcome some of the disadvantages of BOLD-sensitive fMRI for pharmacological challenge imaging. Arterial spin labeling (ASL) is one such method that promises to provide more quantitative measures. Images taken after administration of MRI signal enhancing agents (“contrast”) can be made quantitative under some circumstances. These methods have their own disadvantages including relatively poor temporal sensitivity or the need to administer the contrast agent with potential side effects.
However, even if the specific disadvantages of existing imaging methods for pharmacological challenge imaging are overcome, they do not generate quantitative pharmacodynamic parameters in current implementations. In fact, in order to generate such quantitative data, all current pharmacological activation imaging approaches require repeated experiments, each after a different dose of a drug of interest, to generate accurate dose-response curves or otherwise to estimate quantitative pharmacodynamic parameters such as EC50. These repeated experiments must also include experiments done at relatively high doses of the challenge drug (e.g., those that produce at least 90% of the maximal possible signal, or doses that can be many times higher than the dose that produces 50% of the maximal possible signal).
To summarize, the prior art is characterized by a state in which quantitative measures of pharmacodynamic parameters require one or more of the following undesirable characteristics: (a) invasive measurements; (b) many measurement (e.g. scanning) sessions; (c) a quantitative measure, e.g. a quantitative imaging method; (d) known pharmacokinetics; (e) observable clinical reactions such as pain relief or a drug “high.”
Without quantitative information some types of clinical information and research goals cannot be achieved. The dose of drug needed by a given patient cannot be estimated except to say that a response is or is not seen to a given test dose. Diagnostic tests cannot be done unless the detection threshold for any response to drug happens fortuitously to correspond to the optimal drug dose response to which best separates ill from healthy tissue. Any response to an intervention, other than an “all or none” response, can not be detected. Progression of disease severity over time cannot be monitored.
On the contrary, the present application acknowledges the lack of existing methods to provide quantitative pharmacodynamic information on responses to drugs and simultaneously map those responses in different regions of an organ or organism. It also acknowledges the lack of satisfactory existing quantitative methods for diagnosis or drug dosage determination by imaging. The present application uses specific pharmacological stimuli and non-invasive imaging techniques with high spatial resolution and gives a solution to said research and medical needs.